feat(syntaxes/lean): add highlighting support for binders

[eric-wieser]

The following code:

namespace pi
universes u v w
variable {I : Type u}     -- The indexing type
variable {f : I → Type v} -- The family of types already equipped with instances
variables (x y : Π i, f i) (i : I)

#check λ x /-"foo"-/ y, x + y

instance module (α) [semiring α] {m : ∀ i, add_comm_monoid $ f i}
  [∀ i, module α $ f i] :
  @module α (Π i : I, f i) r (@pi.add_comm_monoid I f m) :=
{ add_smul := λ c f g, funext $ λ i, add_smul _ _ _,
  zero_smul := λ f, funext $ λ i, zero_smul α _,
  ..pi.distrib_mul_action _ }

which previously was highlighted as

Is now highlighted as:

What's new is that vscode now recognizes names within binders.

[eric-wieser]

Otherwise the result is:

Which is a little too green to be useful.

[gebner]

This looks wonderful! Thanks!